Piston and reactance combinations for radial piston pumps and motors



June 27, 1939. K BENEDEK 2,163,570

FISTON AND REACTANCE COMBINATIONS FOR RADIAL PISTON PUMPS AND MOTORS Filed Nov. 11, 1935 WWII/(I14 16 9 6 w" w [F1 in [76 ml V T1535 ELEK KQE] ENED EKQ tons, piston heads and ,guide means.

Patented June 27, 1939 UNITED STATES 2,163,570 PISTON AND REACTANCE COMBINATIONS FOR RADIAL PISTON PUMPS AND TORS I Elek K. Benedek, Bucyrus, Ohio Application November 11, 1935, Serial No. 49,261

9 Claims. (01. 103-161) This invention relates to piston and reactance combinations and particularly to piston head and reactance guide means for the purpose of providing hydrostatic, hydrodynamic, and capillary oil film bearings between the piston heads and the cooperating parts of the reactance guide means so that more efficient combined load and torque transmission is efiected.

In the rotary radial piston type of pump or motor of the general character described in my United States patents, No. 1,964,244, issued June 26, 1934, and No. 2,000,271, issued May 7, 1935, it is necessary to compensate for tangential components of eccentricity between the primary and secondary rotors by provision for relative oscillation of the reactance guide means and the piston head.

In the oscillating type of radial piston pump,

such as described in my United States Patent No. 1,964,245, issued June 26, 1934, provision must be made for relative tangential ,oscillation between the reactance guide means and the piston heads.

In either event, this oscillation is in a plane normal to the axis of the piston. As more fully set forth in my co-pending application, Ser. No. 736,550, filed July 23, 1934, the most eflicient combined load and torque transmission is eifected by combined -mechanical and hydrostatic and hydrodynamic oil film bearings between the piston head and the reactance guide means.

Again, in each of these types of pumps or motors, it is necessary to compensate for manufacturing inaccuracies or' inaccuracies in adjustment of the guide means and piston automatically, so that misalignment from these causes is eliminated and free operation of the piston head and uniform and efficient application of the load and torque is provided, regardless of the relative positions of oscillation of the guide means and the heads. 1 w 1 With these facts in mind, it is one of the objects of the present invention to provide pistons, piston heads, and reactance guide means all so cooperated as to assure proper alignment of and formation of hydrostatic and hydrodynamic oil films between the working surfaces of the pis- Another object is to provide for a more efficient mechanical bearing between the piston heads and guide means in combination with the hydrodynamic and hydrostatic oil film bearings therebetween.

A more specific object is to eifect these results while providing for automatical self adjustment of the coacting parts and resultant compensation for inaccuracies in manufacture or adjustment of the structure by piston and head structures which will be self-adjusting with respect to the reactance surfaces of the reactance guide means and with each other so that the load will be applied axially of the piston at all times.

Another object is to effect these results with structures which may be formed readily and manufactured economically by quantity produc- 0 tion methods and from readily available materials.

Other objects and advantages will become apparent from the following specification wherein reference is made to the drawing, in which Fig. l is an elevation of a piston and piston head structure embodying the principles of the present invention, part thereof being shown in section for clearness in illustration;

Figs. 2 and 3 are an elevation and plan view of a locking and cushioning washer used in connection with the piston and piston head structure in Fig. 1;

Fig. 4 is an elevation of a piston, with-a piston head structure and reactance guide means illustrating a modified form of the invention, part thereof being shown in section;

Figs. 5 and 6 are a plan and elevation respec tively of a piston and piston head structure embodying the principles of the present invention, part of the structure in Fig. 6 being shown in section for clearness in illustration;

Fig. 7 is an elevation of the load transmitting thrust pin of Fig. 6; v

Fig. 8 is an elevation of a piston and head combination, part thereof-being shown in section as taken on the line 8-8 of Fig. 9, and illustrating a modification of the invention;

Fig. 9 is an elevation of the structure illustrated in Fig. 8 and is taken at right angles thereto, part being shown in section.

Fig. 10 is a sectional view of a modified piston, head, and reactance guide means combination, part thereof being shown in section for clearness; and

Fig. 11 is an elevation, partly in section, of a modified piston and head combination corresponding in operation to the structures of Figs. l and 6.

' For the purposes of illustrating the operation of the piston heads, they will be described herein as though associated with rotary radial piston hydraulic pumps and motors, such as described in my above identified patents, Nos. 1,964,244 and 2,000,271, wherein a rotary reactance having piston head 2,- generallyin the form of the usual T-head, is provided and has a central bore 2a internally threaded to receive the threaded portion Ia of the piston I. The bore 2a terminates inwardly of the head 2 in a radial shoulder 2b.

The head is also provided at the face adjacent the piston with a counterbored recess 20 which is of greater diameter than the piston and is coaxial with the bore 2a.

Interposed r between the shoulder lb of the piston and the base of the recess 20 is a resilient cushion 3 which is preferably a steel lock washer, such as illustrated in-Figs. 2 and 3. The threads of the piston and head are preferably standard threads so as to fit with a slight yieldability or play. The cushion or washer .3 is such that when unstressed it is of greater length than the depth of the recess 20 and it is also of smaller diameterthan the recess and substantially the same diameter as theannular shoulder lb. The piston and head are screwed firmly together compressing the spring washer or cushion 3, which, when compressed, substantially fills the recess 20. In this assembled condition the end of the threaded portion Ia of thepiston is in slightly spaced relation to the shoulder 27) of the head. 4

The cushion or washer 3 is provided with knife edges 311' so disposed that when the piston I is screwed into the head, the head 2 andpiston rotate away from the knife edge, but when the piston and head are rotated in the opposite direction, the knife edges will dig into the shoulder usual radial cylinder.

tive to the piston occurs.

' guide means of a rotary reactance which is as- Ib and face of the counterbore end wall and thus prevent rotation and separation of the piston I and head 2. Since the threaded end of the piston does not engage the shoulder 21), the load transmission is through the medium of the spring and shoulder Ib and thus directed parallel to the axis of the piston. Even when compressed in i this manner, the elastic effect of the spring and the mating of the threads of the head and piston are such that slight movement of the head re la- As a result, the head may-bemoved a slight fraction oil/1000 of an inch radially of the piston as well as axially and may rock very slightly about the piston axis.

To illustrate the manner in'which this structure operates, it is shown in connection with the su'medto berotating clockwise. A fragment of a'reactance rotor A, such'as described in my above identified patentsjhaving usual outer and inner chordal guide surfaces B and C respec-- tively, is illustrated, the piston operating in the w The surface B engages the outer or load transmitting face 2d of the head 2 and the surface C supports the piston by en-J gagementwith the inner or suction working face 2e. Sincethe head I is mounted on the piston I for yieldable universal rocking movement and movement radially of the piston, it is self adiustingrelative to thesurfaoes B and cat well as to the piston I, so that an partsoperate in proper relation to each other. Minor inaccuracies .necessarily occur in the r'nanufacture of the reactance and the pistons, as

well as in the cylinder rotor, but none of these are so pronounced that the slight resulting mis-.

alignment is not compensated. In operation, the head 2 oscillates along the surfaces B and C of the guide means A, as fully described in my above identified patents. Duringsuch operation, the head 2 rocks slightly'and yieldably about a transverse axis with the result that wedge shape spaces are formed between the surfaces of the head 2 and the cooperating surfaces of the guide means, these spaces being sumcient to maintain capillary oil films so that combined torque and load transmission is through the medium of high pressure ofl bearings. Likewise, oil is maintained the head and piston. Consequently all pistons of the pump or motor' share evenly in torque transmission,

. by capillarity between the coacting surfaces of trated, the piston having a head 5 which may be formed integrally therewith and which iselongated fore and aft in the direction of oscillation of the head relative to the reactance rotor. The piston head has inner working surface So for engagement with the inner guide surfaces of the associated guideway; Interposed between the outer reactance guideway and the head 5 is a reactance andtorque transmitting interponent 6, which, in effect, is part 'of the piston head structure and has a; guideway engaging, or working, face to for engagement with the guideway.

In order to transmit load from-the reactance means to the piston at asingle point or area at the axis of the piston, the face 5b .of the head portion. 5 and the inner face 62 'of the inter- 'ponent 8 are formed with divergent surfaces, one of the faces being cylindrical and the other being tangential thereto. In the form illustrated,

- the inner face 612 of the interponent is cylindrical about an axis parallel to the axis of'rotation and outer face lb is flat and tangential thereto.

Thus the interponent 6 may rock slightly about a constantly shifting axis which is at all times parallel to the axis of rotation and thereby effect a slight rolling engagement with the outer surface 5b.- This rolling is so slight that the theoretical point of tangency of the cylindrical face 6b andthe face 5b is at all times very close to the axis of the piston. Due to the rolling oscilla- :ion, oil films are formed between all working 7 In order to transmit torque through the medium of the head structure, the interponent 6 is provided at eachend of its cylindrical surface with depending transverse shoulders 60. Correspondingly, the head 5 is provided with cooperating shoulders 5c, the shoulders 8c and 5c engageach other for transmitting torque while perinitting slight rolling oscillation ofthe inter,- pon'ent relative to the head So. ing between the shoulders 5c and 80,; the shoulders Ic are slightly undercut so that only the edges thereof in the plane of the face 5b engage the shoulders to. a 1

Thus, asmning the interponent 6 as apart of thepistcn head structure, it is apparent that hydraulic load is applied at the axis of the piston To prevent bind- I with slight or rolling oscillationi'oricom- 15 pensating for inaccuracies and maintaining proper oil films both in the structures of Fig. 1 and Fig. 4.

Another manner of accomplishing this same result is illustrated in Figs. 5 to 7 inclusive, wherein isv illustrated a piston I having a head portion 8 which may be formed integrally therewith and which is elongated in the direction of oscillation ,of the head in the reactance means. The head has the usual inner working or suction face 8a. At its outer, or load, face the head 8 is recessed, as indicated at 8b, the recess 8b extending in the direction of oscillation of the head and terminating short of the ends thereof.-

The end walls 8c of the recess provide "torque 1 transmitting abutments. The head portion is also provided'with an axial bore 811 extending from the base of the recess inwardly and having a generally spherical end wall 8e. Disposed the limits of the recess 81). An interponent I0 forming the remainder of the head structure and having an outer working or load face- Illa for cooperation with the reactance guideways is provided, the interponent I0 having a thickened central portion Illb defining with the end poractance guideways.

tions transversely extending torque transmitting shoulders lllc. The inner face IIId of the interponent engages the outer end of the pin 9 and when so engaged the shoulders Ilic are in position to abut the shoulders 8c for torque transmission. Slight clearance in the direction of travel is provided between the shoulders I00 and 8c so that the interponent I0 can become self aligning. Thus the load is transmitted through the spherical end surfaces of the pin 9 and axially of the piston I regardless of the rocking action of the interponent II) which necessare ily oscillates by rolling action about the outer end of the spherical pin 9. Oil is retained in the recess 8b so that combination oil and mechanical bearing is obtained.

Referring next to Figs. 8' and 9. a similar operation is obtained with a piston II having a head I2 elongated in a direction of oscillation. The head portion I2 is preferably formed integral with the piston -II and has an inner chordal working face I2a for engagement with the re The outer working face 121) of the head portion I2 is preferably spherical or spheroidal, the working surface thus being arcuatein crosssection in every direction from the axis of the piston. At the ends of the spherical surface I21), forwardly and rearwardly in the direction of .oscillation, are abutment shoulders I20 for transmitting torque. An interponent I3 forming a part of the piston head structure and having a spherical inner workin'g'face I3a for engagement with the face I2b of the head is provided. At the ends of the spherical face lid, in

the direction of oscillation of the head in the are depending transverse shoulders I3b which cooperate respectively with reactance rotor.

the shoulders I2c for effecting torque transmission. The outer face I3c of the-interponent arranged to cooperate with the outer chordal guideway of the rotary reactance. For reducing frictional resistance to oscillation, the face I30 may be recessed,-as indicated at I3d, for accomverse ax s.

modating a plurality of capillary cageless needle rollers I4. Thus limited universal rocking of the has an outer cylindrical or spherical working face I60. and an inner cylindrical or spherical working face ISb, the faces being cylindrical or spherical about axes parallel to the axis of rotation. In this structure a pair of interponents complete the head structure. The interponents include an inner interponent II having a central passage for accommodating the piston. The

interponent II has an "inner working face I'Ia adapted for engagement with the inner chordal guide surface of the reactance guideway. The outer working face III) of the interponent II is preferably flat so as to define with the face' I6b wedge-shaped spaces for receiving lubricant. An outer interponent I8, having a flat outer working face I8a for cooperation with the'outer chordal guide surface of the reactance guideway is also provided. The inner face IBI) of the interponent i8 is likewise flat so as to provide theoretical line contact between the outer face I6a of the piston head portion 16 and the inner face of the interponent I8. In th s head structure. the interponent I8 rocks about an axis parallel to the axis of rotation and about a trans- Suitable end shoulders IBc are formed on the interponent I8 for engagement with complementary shoulders IE0 at 'the leading and trailing edges of the head I6 for effecting proper torque transmission. Due to the rocking action, oil films are positively maintained by hydrodynam c action as well as by capillary action.

is illustrated. The piston has a longitudinally elongated head portion 20, the head 20 having an inner working face 20a for engagement with.

the chordal guideways of the reactance means, as

illustrated. The head 20 has an axial bore 20b,

the bottom of which is spherical. An interponent 2|, forming the other part of the head.

structure cooperates with the head portion 20, the interponent having an outer working face 2Ia for engagement with the outer chordal guideway of the reactance means. The interponent likewise has an inner face 2" which preferably is slightly cylindrical about an axis parallel to the axis of rotation or isslightly tapered from the central portion toward each end. At-its centralportion, the interponent 2| has a depending ball portion 22 which may be formed integral therewith, the portion 22 being snugly received in the bore 20b of the p ston so as to form'a ball and socket load transmission connection. In thisstructure the in erponent 2! may rock with universal rolling action due to the ball and socketconnection and is also effective for torque transmission. By makng the bore 20b of slightly larger radius than the ball portion, rolling action Slight capillary clearance is also provided between the shoulders of the various structures so as to permit the entrance of slip fluid and the forma-- tion 'of capillary oil films. -The various parts of each structure are preferably accurately and smoothly finished and polished and hardened to so that each chordal way is fixed circumferentially r and angularly relative to the other chordal ways.

In connection with such a reactance any misalignment of the reactance surfaces and piston head structures must be remedied in the piston head structures or by actual re-cutting of the reactance ways.

Four points of misalignment must be considered. The first two are that the outer or inner Y reactance surfaces, or both, of each way may not be exactly 90 to the associated piston axis, due to inequalities in spacing the cylinders exactly the same distance apart circumferentially of the barrel or due to inaccuracies in forming the cylinders truly radial. The third point 'is that the inner and outer reactance surfaces of a given chordal way may not be truly parallel with each other. A fourth point is that either the inner reactance surfaces or theouter reactance surfaces may not be in the same plane with each other or in a plane transversely normal to the piston axis.

The cooperation of the present head structures with the reactance of Patent No. 1,964,244 should be noted briefly. If the structure of Fig. l is utilized, obviously, if the irmer and outer re' actance surface C and B are parallel, the surfaces 2d and 2e can align therewith whether they define'planes normal to the piston axis or not as the head can rock both fore and aft and'transversely. If,.however, the coacting inner and outer surfaces of any way are not parallel, they must be reformed for parallelism.- If one way is at a slightly different distance radially of the reactance from its laterally-aligned coasting way, the head can rockaboutfa longitudinal axis and largely correct this inaccuracy. If the cylinder is slightly out of position circumferentially, the head can rock about a transverse axis to compensate.

If thestructure of Fig. 4 is utilized, since the head 5 is rigid with the piston, positive alignment must be made between its surface in. and the inner reactance surfaces of the chordal way, even 'if reforming the reactance surfaceis required.

and regardless of the cause of inaccuracy. This being done, the head portion v8 rocks to permit alignment of face 3a with the outerreactance surfaces. If the laterally aligned chordal ways for the particular piston head are not truly in a plane trated in Figs. 6, s and a, and m. 1o, whefieln the normal to the piston axis, the surfaces thereof which enact-with the fixed portion 'of the piston must be reformed to eliminate this inaccuracy.

Such an inaccuracy in the reactance surfaces cooperating with the movable head portion may becompensated, however, by structures such as illusmovable head portions Ill, l3 and 2|, respectively, can rock universally to effect self-alignment.

The structure of Fig. 10 has very special advantages in connection with this particular type of reactance structure. It will be noted that both the head portions l1 and I8 are independently self-adjusting so that, regardless of the inaccuracies in the ways, as above described, no portion'of theways needs to be reformed. Instead,

each head portion readily adjusts itself to its particular reactance surfaces of the ways and of the head.

Referring next to the type of reactance illustrated in my United States Patent No. 2,000,271, the reactance is comprised of segments which are circumferentially adjustable with respect to each other, but the inner and. outer reactance surof the ways. Thus fore and aft, the ways can be adjusted to 90 to the cylinder axes and properly lined. up in this respect with the rigid piston head portions. The movable head portions can then compensate for inaccuracies in the outer reactance surfaces of' the ways. The structure of Fig. 10, when used in this reactance, is very beneficial also, if most extremely small inaccuracies are to be compensated, though ordinarily, the circumferential adjustment of the reactancesegments render unnecessary the joint use of the head members I! and I 8.

Referring next to the reactance structure of my copending application 736,550, there is illustrated areactance formed-'of circumferentially adjustable segments such as that of my United States Patent No,2,000,271 with the additional feature that one reactance surface of each guideway is provided by a circumferentially movable block such that the reactance surfaces of each way are self-adjusting into parallelism with respect to each other or to the piston heads. This latter Y reactance may be used with each structure herein illustrated, and is particularly useful for great precision, Since, in each chordal way, inaccuracies in parallelism of the inner reactance surface with the outer are compensated, it is especially .useful in Connection with the structure of Fig. 1

herein, wherein the outer and inner faces of the head 2 are fixed 'in relation to each other, Thus by circumferential adjustment of the re'actance segments, the inner surfaces of the ways-can be brought into accurate coaction with, the inner surfaces of the head 2, and the circumferentially movable blocks can adjust, themselves to the outer faces of'the heads. The use of this reactance which has .dual adjustment featuresin con.

nection with the other .head structures herein disclosed provides for extreme accuracy in oooperation. By use of the various combinations of the-reactance herein described with the piston head structure, it is apparent that almost any Specific self-adiusting problem can be solved and any degree'of precision of cooperation obtained.

The reactance' portions herein shown diagrammatically may be each of the-types of reacta'nces described in the above identified patents and ap plication, or other types employing chordal reactance ways. It is apparent that in each structure, the load is transmitted substantially at and parallel to the axis of the piston, the point of application of the load being automatically maintained by the self-adjustment and self-alignment of the pistons, head structures, and guideways. Since proper alignment of the head structures with the reactance rotor guideways is effected,

. each piston may transmit its proportionate part of the torque.

As heretofore pointed out, the and outer are merely relative terms for convenience in description, as obviously the same operative relation can. be maintained when the piston heads are connected to the inner rotor, or when the cylinders are the reciprocating members.

Having thus described my invention, I.c1aim:

1. A piston and head structure ,for a radial piston pump or motor having a pair of rotors corotatable about parallel axes, one rotor having a radial cylinder, a piston therein, inner and outer guideways respectively in the other rotor extending normal to the cylinder axis for accommodating the piston head structure for oscillation therealongand for cooperating therewith to transmit hydraulic load and torque, a head portion on the piston extending normal to the piston axis and having a working face for cooperation with' one of said guideways, a separate head portion having an oppositely disposed working face for co- 7 operation with the other of said guideways, said head portions having space-defining faces con-' tiguous to each other between said inner and outer guideways, and said contiguous faces conjointly being so formed as to provide substantially wedge-shaped spaces extending inwardly from the edges of the head portions toward the piston axis, one of said head portions having a bore centrally disposed therein coaxial with the piston and extending into said head portion well belowthe space-defining face thereofyand an axial thrust element received in said bore and operatively connected to the other one of said head portions for load transmission between said portions and for compensating for misalignment of said cylinder bore and guideway.

2. A piston and head structure for a radial piston pump or motor having a pair of rotors corotatable about parallel axes, one rotor having a radial cylinder, a piston therein, guideways in the other rotor extending normal to the cylinder axis for accommodating the piston head structure for oscillation therealong and for cooperating therewith to transmit hydraulic load and torque, comprising a head portion on thepiston extending normal to the piston axis, another head portion movable relative to the piston, said head pore tions having space-defining faces contiguous to each other. between said guideways and being conjointly formed to provide space between at least the marginal portions of the contiguous faces and one head portion having a socket of less diameter than the piston and coaxial therewith, a

ball on the other head portion'and seated in the guideways, whereby free reciprocation of the terms inner axis for accommodating the piston head structure for oscillation therealong and for cooperat ing therewith to transmit hydraulic load and torque, comprising an elongated head portion rigid with the piston and extending normal to the piston axis, a cooperating head portion generally normal to the piston axis and movably relative to the piston, said portions having coacting surfaces in rolling engagement with each other and working faces respectively in cooperation with said guideways, means'carried by the head portions in addition to said rolling surfaces and operatively engaging each other for constraining the head portions to limited relative movement endwise of the head portions.

'4. A piston and head structure for a radial piston pump or motor having a pair of rotors corotatable about parallel axes, one rotor having a radial cylinder, a piston therein, innerand outer guideways in the other rotor extending.

normal to the cylinder axis for accommodating the piston head structure for oscillation therealong and for cooperating therewith to transmit hydraulic load and torque, comprising a. piston having a head portion fixed to the piston and extending normal to the piston axis, said head portion having a working face for cooperation with one of said guideways, another head portion having an oppositely disposed working facefor cooperation with the other of said guideways, and being movable relative to the piston, one of said head portions having a central bore coaxial with the piston, an axial thrust pin received in said bore and operatively connected to the other one of said head portions for load transmission between said head portions and for connecting the portions for universal relative rolling rock-' ing movement, cooperating abutment meanscarried by the head portions in addition to said pin for constraining the movable head portion within certain limits endwise .of the fixed head portion while permitting rocking of one of the head portions relative to the piston to effect self-alignment of the last mentioned head portion with the guideways.

5. A piston and head structure for aradial piston pump or motor having a pair of rotors corotatable about parallel axes, one rotor having' a radial cylinder, a piston therein, inner and outer guideways in the other rotor extending'normal to the'cylinder axis for accommodating the piston head structure for oscillation therealong and for cooperating therewith to transmit hydraulic load and torque, comprising a head portion fixed in relation to the piston and in engagement with one of said guideways, a movable head portion in load transmitting cooperation with the fixed head portion and rockable relative thereto with a rolling motion for self-alignment thereof with the other of said guideways, and cooperating shoulders on said portions extending transversely relative to the path of oscillation and engageable' with I 70 each other and constraining the portions within certain limits of movement'parallel to the guideways with respect to each other.

6. A piston and head structure for a radial piston pump or motor having a pair of rotors corotatable about parallel axes, one rotor having a radial cylinder, a piston-therein, inner and outer guideways in the other rotor extending normal to the cylinder axis for accommodating the piston head structure for oscillation therealong and for cooperating therewith to transmit hydraulic load and torque, comprising a head.

portion fixed in relation to the Piston, a movable head portion in load transmitting cooperation with the fixed head portion and rockable relative thereto with rolling motion for self alignment.

thereof relative to each other parallel tothe 7. A piston and head structure for a radial piston pump or motor having a pair of rotors corotatable about parallel axes, one rotor having a radial cylinder, a piston therein, guideways in the other rotor extending normal to the cylinder axis for accommodating the piston head structure for oscillation therealong and for cooperating therewith to transmit hydraulic load and torque, comprising a head portion fixed in relation to the piston,-a movable head portion in load transmitting cooperation with the fixed head portion and rockable relative thereto for self-alignment thereof with the guideways, and cooperating shoulders on said portions engageable with each other and constraining the portions within certain limits of movement parallel to the guideways with respect to each other while permitting relative rocking oi.' said portions.

8. A piston and head structure fora radial piston'pum'p or motor having a pair of rotors corotatable about parallel axes, one rotor having a radial cylinder, a piston therein, guideways in the other rotor extendingnormal' to the cylinderaxis rections means carried by one of the reciprocating elefor accommodating the piston head structure for oscillation therealong and for cooperating therewith totransmit hydraulic load and torque, comprising a head portion connected to the piston and extending generally normal to the piston axis, a head portion operatively connected to the defining faces contiguous to each other between said guideways and being conjointly formed to provide space between at least the marginal portions of the contiguous faces and said head por .movement parallel to the guideways.

'9. A pumping unit comprising a pair of corotating guide members, reciprocating elements carried by each of said members respectively and reciprocable relative thereto respectively, means on said members constraining the elements respectively to reciprocation in predetermined diconcurrently respectively, separate ments and guided in one of the guide members, said means including a thrust plate and a central pivotal means projecting from said thrust plate and terminating in inwardly spaced relation from the sides and ends of said last-named element,

there being space between the respectively contiguous sides and ends of said element and said thrust plate, said pivotal means being disposed in the axis of the other reciprocating element to transmit force between said other element and thrust pla'tethrough substantially point contact, and mounting the elements for rol1ing rocking movement relative to each other;

first head portion for load transmission axially of the piston, said head portions having space-.

ELEK K. BENEDEK. 40- 

